Many conventional filter structures include a porous membrane as the filter medium, combined with non-porous structural components, in order to obtain a filtering element sufficiently rugged to withstand high differential pressure caused by flow of fluid in one direction; however, such structures are poorly suited to conditions of bidirectional flow. Bidirectional flow is often necessary, for example, to remove collected solids from the surface of the filter.
To provide the requisite strength for bidirectional flow, the filter structure may include a stronger support structure to which the membrane is physically attached or bonded, e.g., a support structure such as a core in a cylindrical filter element. This support structure should preferably not interfere with the filtering functions of the membrane while lending the membrane requisite support.
Filter structures of this general type have several drawbacks. For example, cylindrical filter elements with the filtration membrane mounted on the exterior of a grooved and/or perforated cylindrical core support function effectively provided flow is from the outside to the inside. However, if reverse flow occurs from inside to outside, as often happens in practice, the membrane is prone to rupture with catastrophic results. For example, a typical 0.15 centimeter (cm) thick membrane wrapped and edge sealed about a cylindrical support core 7.5 cm in diameter and exposed to a relatively low reverse differential pressure of 0.7 kilograms per square centimeter (or 10 pounds per square inch, or psi) experiences a stress of about 170 kilograms per square centimeter (or about 2,500 psi), which would cause even the most rugged membrane to fail. Similarly, if the cylindrical filter element or structure is rapidly rotated about its longitudinal axis with concomitant high centrifugal forces, the membrane may rupture.
Various techniques have been developed in an attempt to overcome these types of problems; attaching the membrane to the support by melt bonding is one approach. However, the cost of labor to perform the necessary procedure is, for many purposes, excessive, and most or all such procedures prevent flow through the attached areas. Furthermore, the necessity for the core support to be formed of low melting plastic, when melt bonding is used to secure the membrane, limits the useful temperature range of such filters. Another disadvantage is the use of different kinds of resin for the membrane and the support, each with its own vulnerability to chemical attack, thereby limiting the range of applications compared with that of structures in which the support and the microporous structure are both formed of the same resin, for example, if both are the same polyamide. The problem is exacerbated if an adhesive of still another composition is used to secure the membrane to the support.
Additional problems may arise with systems using an adhesive. If too little of the adhesive or solvent is added, the membrane is only weakly bonded to the support structure. If too much adhesive or solvent is applied, the membrane may be blinded by the excess adhesive or solvent, decreasing the filtering capacity of the membrane. Even if a very thin coat of adhesive is applied, essentially all of the adhesive tends to be absorbed into the pores of the membrane, causing weak or zero bonding. It is also possible that the fluid being filtered is incompatible with the adhesive or that the adhesive will leach into the fluid being filtered and contaminate it.